The shaft type/size of a helical pile is critical to both the axial and lateral capacity – especially for compression in soft/loose overburden soils where lateral stability of the shaft must be considered. The following is a brief summary of the 4 different shaft types commonly used for helical piles and their relative advantages and disadvantages based on site conditions and application. It is very important to understand that helical pile installation must be considered in the design process.
There are several solutions engineers and contractors can choose from when a deep foundation is required. With a deep foundation the structure’s load is transmitted to soils that are deeper in the ground. A deep foundation is used when a shallow foundation is not possible, not practical, or will not carry the load. Examples are weak, unstable, or expansive surface soils. Two popular options for deep foundations are helical piles and drilled shafts, also known as drilled piers or caissons.
The first question we often get is, “are helical piles a new technology?”. Though they are growing each year in use and popularity, many Geotechnical and Structural Engineers are often unfamiliar with the technology. Complicating the matter is that, in many cases, students who will be our future engineers know very little about helical piles and their history. The truth is that helical piles [a.k.a. screws, screw piles, helical piers, helicals] appeared on the scene in the early 1800s. Alexander Mitchell (1780-1868) applied the use of screw piles in 1836 for moorings and, in 1838, for a lighthouse foundation. To put things into perspective, screw piles preceded the advent of Portland Cement [1850’s] and first use of a gasoline-powered automobile . According to some historians, screw piles were the major foundation technology of the 19th century.
Any contractor can tell you that a structure is only strong as its foundation. Deep foundations transmit the structure’s load to soils that are deeper in the ground. A deep foundation is used when a shallow foundation is not possible, not practical, or will not carry the load, such as in weak, unstable, or expansive surface soils. Proper site preparation and installation is key to the success of any job – starting with the foundation.
Deep foundation pricing is multifaceted and a one price fits all approach can lead to unexpected costs. To break down the pricing, the following list can help provide a better understanding of the top 4 factors that can affect pile prices and why they are important pieces of information when looking for estimates:
A key benefit of helical piles is the ability to install them in environmentally sensitive areas. CHANCE helical piles are precisely engineered to create minimal disturbance to the soil itself during installation with the pitch and placement of the helices precisely calculated. Helical piles can also be installed using limited access installation equipment eliminating the need for damaging access roads and large equipment required for other foundation types.
Helical piles have become a common piling system in the North American construction market. They are used for many applications with low to moderate tension and compression loads. Slender helical piles, less than 4.5 inches in diameter, have historically been used for light shear loads of 1 to 5 kips in vertical applications. This method derives the lateral capacity as a function of the pile’s allowable structural bending moment and the allowable passive earth pressure of the soil. It is common to analyze the capacity of the piles by theoretical slender pile equations, P-Y curves, or software programs that model P-Y response. An adequate safety factor or a service limit on the allowable deflection is applied to the allowable loads to determine an applicable pipe size. In recent years larger piles, with diameter greater than 5.5 inches, have been used for larger lateral loads. Large diameter piles have been designed for loads in excess of 20 kips.